A qualitative description of preferred orientation in porous carbonate matrices of marine origin

Porous aragonite matrices of marine origin exhibit a prominent preferred orientation in which the (221) crystal planes are aligned perpendicular to the principal growth direction of the organism. Since the aragonite crystallites that compose the matrix appear to be organized perpendicular to the spherical pore surfaces, these apparently conflicting findings can be explained by a bimodal distribution of the crystallites into pore and bulk crystallite populations. Analysis of X-ray diffraction data obtained from matrices taken from eight different organisms was carried out. The validity of the bimodal distribution model was confirmed by correlation with porosity data.